In this new proposal, we investigate our findings defining novel growth factor combinations and signaling pathways that control EC tubulogenesis and EC-pericyte tube co-assembly, which are necessary to create capillary networks, which critically support tissue perfusion, development and functional maintenance. Capillaries consist of two major cell types, ECs and pericytes, which co-assemble to form polarized EC-lined tubes with abluminally positioned pericytes and an intervening basement membrane matrix. The Davis lab has pioneered the development of in vitro systems using human ECs and pericytes, which model these events in 3D matrices under serum-free defined conditions. Remarkably, we have identified that addition of five defined growth factors, SCF, IL-3, SDF-1a, FGF-2, and insulin (Factors), allows for human EC tubulogenesis and recruitment of pericytes with accompanying vascular basement membrane matrix assembly, a finding first reported by our laboratory. Novel preliminary data reveals that these Factors and their receptors control tubulogenesis through activation of three key synergistic signaling pathways; PI3K/Akt/mTor, Raf/Mek/Erk, and Jak/Stat. These pathways are strongly activated and sustained over time by the Factors in a manner that is unique to them, and which are necessary for EC tubulogenesis; and this important signaling is not manifest following VEGF, FGF-2 or VEGF+FGF-2 addition. Furthermore, our defined system led to another major insight demonstrating a novel role for VEGF as an upstream primer of Factor-induced EC tubulogenesis and EC-pericyte tube co-assembly, while it fails to stimulate these processes directly. We also reported that EC- derived PDGF-BB and HB-EGF play a key role in mediating pericyte recruitment, proliferation and basement membrane formation, an observation that was confirmed in vivo using developing quail embryos. Finally, we have obtained novel evidence for a role for EC-derived TGF1, activinB, CTGF, and CCL2 family chemokines in EC-pericyte tube co-assembly. Together, these new insights provide a molecular road map to dissect how defined growth factors, receptors, their common adapters, and overlapping signaling pathways control EC tubulogenesis and EC-pericyte tube co-assembly using both in vitro and in vivo experimental approaches. We propose three specific aims to further investigate these novel insights into the fundamental process of capillary tube assembly in vitro and in vivo in response to defined growth factors and they are; Aim1: To identify and characterize required SCF, IL-3, SDF-1a and FGF-2- and receptor-dependent signaling events that control EC tubulogenesis in vitro and in vivo. Aim2: To elucidate the underlying mechanisms how Factor receptors and adapters synergistically work together to activate the signaling pathways, Akt/mTor, Raf/Erk, and Jak/Stat, leading to EC tubulogenesis. Aim3: To elucidate how SCF, IL-3, SDF-1a and FGF-2- and receptor-dependent signaling leads to EC- pericyte tube co-assembly and vascular basement membrane formation.